Quadrature couplers are indispensable microwave components. They are used in phase shifters, balanced amplifiers, mixers, baluns and other microwave circuits. Basically, a coupler splits equally or unequally, microwave or RF signals into two output signals having a 90 degree phase difference. Many of these applications require 3 dB couplers which are traditionally realized using tightly coupled interdigitated multi-conductor microstrip lines, such as the Lange coupler. See an article entitled "INTERDIGITATED STRIP LINE QUADRATURE HYBRID", published in the IEEE Transactions On Microwave Theory Tech. Vol. MTT-17 December 1969, pages 1150-1151 by J. Lange. The coupler described in that article is referred to as the Lange coupler. See also an article entitled "GaAs MONOLITHIC LANGE AND WILKINSON COUPLERS", by R. C. Waterman, Jr. et al., published in IEEE Transactions Electron Devices, Vol. ED-28, pages 212-216, February 1981. In these structures, the conductor widths and the spacings between the coupler's conductors can be produced with standard thin film manufacturing processes on thick low-dielectric constant substrates (&gt;250 .mu.m, .epsilon..sub.r &lt;10). However, on thin GaAs substrates used for monolithic microwave integrated circuits or MMICs (75-125 .mu.m, .epsilon..sub.r =12.9), tightly coupled structures are difficult to realize. For example, a process with a minimum line width of 8 .mu.m and a minimum spacing of 8 .mu.m cannot be used to fabricate a 3 dB Lange coupler on a 75 .mu.m thick substrate because dimensions of approximately 4 .mu.m are required. Other techniques such as broadside coupled lines and semi-reenterant sections have been proposed as alternative techniques to achieve tight coupling with reasonable manufacturing tolerances. See for example an article by J. S. Izadian entitled "A NEW 6-18 GHz, -3 dB MULTISECTION HYBRID COUPLER USING ASYMMETRIC BROADSIDE, and EDGE COUPLED LINES", published in the 1989 IEEE MTT-S Digest, pages 243-247. See also an article entitled "A QUASI-TEM DESIGN METHOD FOR 3 dB HYBRID COUPLERS USING A SEMI-REENTRANT COUPLING SECTION", published in the IEEE Transactions on Microwave Theory Tech., Vol. MTT-38, No. 11, November 1990, pages 1731-1736.
These coupled line structures require an extra dielectric layer, usually polyimide, whose thickness must be adjusted to control the coupling factor and does not allow other structures on the same substrate to use different coupling factors. Couplers using microstrip are of great interest because they are compatible with microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs). As indicated, 3 dB broadband couplers are extremely difficult to fabricate using microstrip on thin substrates because of the tight mechanical dimensions. This is especially true on thin GaAs substrates (3 mil thick) for power applications. Such couplers are exceptionally difficult to fabricate and many designers have been looking for other alternatives. As indicated above, the most common technique which mitigates the tight tolerances is the use of interdigitated structures.
It is an object of the present invention to provide a coupled line structure that employs embedded microstrip lines to achieve extremely tight couplings on thin substrates as, for example, on gallium-arsenide substrates. Additionally, the coupled line structure allows each component on the same integrated circuit to use different coupling factors.